The use of voltage sense amplifiers with hysteresis for noise rejection is known. The simplest voltage sense amplifier is an operational amplifier in a positive feedback configuration. In the case of voltage hysteresis two different trip points (Tph and Tpl) are defined and circuits are designed such that when a high signal is to be recognized it must exhibit a voltage higher than Tph before it is recognized and declared a high signal. In a similar manner, before a low signal is recognized it must exhibit a low voltage lower than the second trip point Tpl. A simple illustration of this is provided in FIGS. 1A, 1B, and 1C.
In the quest for higher speed signaling it has recently been proposed to use current mode interconnections rather than voltage mode. The goal is to provide impedance matching on signal interconnection lines to reduce or avoid reflections and ringing on the lines. The technique proposed is matching termination of the signal line(s) to the signal receiver by using current mode interconnections and current mode sense amplifiers or current mode comparators. Signal interconnection and clock distribution lines with low controlled impedances are most amenable to current mode signaling. Metal lines separated from metal ground planes or metal power supply distribution planes (which are at AC ground) by oxide or other integrated circuit insulators will have low characteristic impedances of the order 50 or 75 ohms. To avoid reflections and ringing these need to be terminated in their characteristic impedance which requires sense amplifiers or receivers with low input impedances and implies small voltage swings on the lines. This is most easily accomplished by using current sense amplifiers which normally have a low input impedance. Rather than trying to sense the small voltage swings on the lines one can instead sense the current signal. Both single ended and differential configurations are possible. Current sense amplifiers have been described for use in SRAM's and in low impedance current-mode interconnections in CMOS integrated circuits with shielded interconnection lines. While this will reduce reflections and ringing it will not completely eliminate them. Also, this technique is still susceptible to noise transients.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, it is desirable to develop sense amplifiers or current comparators which are even less susceptible to induced noise, current reflections or ringing.